In the design of the microcontroller for integrated circuits, the oscillator is typically integrated into the microcontroller design on the same chip to accommodate applications wherein space, size and current draw are critical parameters. These integrated oscillators, when operating in the megahertz range, most commonly utilize a pair of CMOS inverters. One inverter is utilized as the oscillator gain element and the other as a buffer. This type of oscillator has some disadvantages in that the inverters operate in a linear region at all times, thus drawing significant current during most of the oscillation cycle. The current draw of such a circuit is proportionate to the strength of the transistor, which is dependent on production, temperature and supply voltage by a factor of approximately 10-to-1. Additionally, these oscillators tend to operate from rail to rail, such that the frequency spectrum output therefrom is fairly "rich."
One solution to the above-noted problem of high current draw is to utilize an NMOS common source amplifier with a current source load. This circuit, often referred as a Pierce oscillator, yields high gain with relatively low and constant power. The output buffer for this type of oscillator is a comparator circuit with one input tied to the input of the amplifier and the other input tied to the output of the amplifier. This allows the amplitude in the oscillator to remain small, while the output of the comparator is capable of switching external circuits quickly, keeping such circuits out of linear region. Optimization of the circuit for any crystal resonant frequency requires only the adjustment of the values of the gate-to-source capacitor and the drain-to-source capacitor.
Whenever an external resonator is the only external device associated with the oscillator, and the circuit is required to perform over a wide range of clock frequencies and resonator types, problems can arise. In most integrated circuit applications, the oscillator circuit is optimized to run over a relatively narrow frequency range, with the frequency of oscillator set by an external quartz crystal. However, when a customer requests a new version of the circuit, or a different clock frequency, this usually requires the oscillator circuit to be redesigned. Such oscillator circuits can require radical changes, this often resulting in longer product redesign cycles, extensive simulation and optimization, new test techniques, and larger die size requirements.